You can play a torrent while it is still downloading. To do this, go to the torrent list view, then tap on the torrent you wish to play. This will land you on the Files page for your torrent. Tap on the icon of the file you wish to play or open.
You may be prompted to choose a player to open the file. Note that in order to play media files, you will need to find and install a media player in the Play Store that is compatible with the file you downloaded. Many phones come with media players installed, so you may not need to download one.
Tip: If you are having trouble, search for the extension of the file you are trying to play, then search the Play Store to find the top-rated app that supports your file.